Which feature of a screw primarily enables its ability to convert rotational motion into linear motion?

• A. The helical thread
• B. The gear ratio
• C. The cam profile
• D. The straight shaft

Answer: (A)

The helical thread is what lets a screw turn rotational motion into linear motion. The thread behaves like an inclined plane wrapped around a cylinder, so when you rotate the screw, the nut threads ride along the helix and are driven forward or backward along the axis. How far they move with each full turn—the pitch (or lead in multi-start screws)—determines the linear displacement per revolution. This coupling of rotation to straight-line movement is the essence of how a screw converts motion. The other features describe different mechanisms—gear ratio governs speed and torque in gears, a cam profile shapes a follower’s motion from rotation, and a straight shaft merely transmits rotation without producing axial movement.